The demand for electrical power supply at the sea floor increases with the increasing water depth at which oil production is being performed. This means that electrical energy must be supplied through power cables. These power cables have to hang freely suspended from the floating production vessel and down to the seabed, i.e. so-called dynamic cables.
Copper is the most common metal used in electrical conductor element. Although having excellent electrical properties such as high conductivity, copper does not have mechanical properties suitable for withstanding the loads imposed during cable installation and during dynamic service, facing the motions induced by wind, currents and waves, and also the high external pressure at the seabed.
Copper has a high density and a low mechanical strength. The high density indirectly leads to large inertia forces during installation and dynamic service.
The low mechanical strength implies that copper will not contribute much to the cable's overall strength or axial stiffness. Furthermore, copper also has a relatively small acceptable maximum strain limit as well as strain range to operate within during dynamic service.
In the existing power cable technology, several conductor elements with a copper core are wound around each other in a bundle surrounded by a number of load bearing armor layers. The load transferring mechanism from each conductor element to the load bearing armor layers is internal friction, which is an unreliable servant.
Moreover, the copper core is classically made of stranded copper wires. Therefore, when a conductor element is subjected to relatively high tensions, contact forces between the copper wires will also be relatively high. Such high contact forces and relative movement between copper wires may cause fretting to occur. And copper has relatively low fretting resistance.